Liquid crystal display device including thin film transistors having gate electrodes completely covering the semiconductor

ABSTRACT

There are disclosed various types of TFT active matrix liquid crystal display devices and method of fabrication thereof in which a pixel is divided into three parts, a capacitor is added to each pixel, light shielding is applied to each TFT, and the matrix is driven by a DC cancelling technique.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/192,313, filed Nov.16, 1998, now U.S. Pat. No. 6,184,963; which is a continuation ofapplication Ser. No. 08/924,737, filed Sep. 5, 1997, now U.S. Pat. No.5,838,399; which was a continuation of application Ser. No. 08/610,148,filed Feb. 29, 1996, now U.S. Pat. No. 5,708,484; which was a divisionalof application Ser. No. 08/457,577, filed Jun. 1, 1995, now U.S. Pat.No. 5,532,850; which was a divisional of application Ser. No.08/277,434, filed Jul. 18, 1994, now U.S. Pat. No. 5,528,396; which wasa divisional of application Ser. No. 07/910,455, filed Jul. 8, 1992, nowU.S. Pat. No. 5,331,447; and which, in turn, was a continuation ofapplication Ser. No. 07/205,185, filed Jun. 10, 1988, now U.S. Pat. No.5,132,820, the entire disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally directed to display devices and, moreparticularly, to active matrix liquid crystal display devices in whichpixels (e.g., picture elements or picture cells) are formed by use ofthin film transistors and pixel electrodes.

2. Description of the Invention

An active matrix liquid crystal display device includes a liquid crystaldisplay unit on which a plurality of pixels are arranged in matrix form.Each individual pixel on the liquid crystal display unit is disposed ineach of intersection regions defined by two adjacent scanning signallines (gate signal lines) and two adjacent image signal lines (drainsignal lines). The plurality of scanning signal lines extending in therow-direction (horizontal direction) are arrayed in thecolumn-direction, while the plurality of image signal lines extending inthe column-direction (vertical direction), intersecting the scanningsignal lines, are arrayed in the row-direction.

The pixel is formed mainly of a liquid crystal in combination with athin film transistor (TFT), a common transparent pixel electrode and atransparent pixel electrode which are disposed through the liquidcrystal. The transparent pixel electrode and the thin film transistorare each provided for every pixel. The transparent pixel electrode isconnected to a source electrode of the thin film transistor. A drainelectrode of the thin film transistor is connected to the image signallines, while a gate electrode is connected to the scanning signal lines.

A typical arrangement is such that unnecessary incident light emergingfrom a panel front surface is shielded by a light shielding film formedon the upper portion of TFT, and beams of backlight which are notrequired are shielded by the non-transparent gate electrode. Inaccordance with a variety of experiments performed, the presentinventors have found that sufficient light shielding effects cannot beobtained by a TFT gate electrode of an ordinary size.

When the light strikes upon an amorphous semiconductor layer of the thinfilm transistor, electron-hole couplings are generated, therebydeteriorating OFF-characteristics of the transistor. Hence, it isrequired that the amorphous semiconductor layer be arranged so as not toundergo the irradiation of light as much as possible. The light fordisplay is classified into two types: natural incident light (or lightof a room lamp) emerging from the front surface of the liquid crystaldisplay panel and incident backlight of a fluorescent lamp which emergesfrom the underside of the panel.

The above-described liquid crystal display device tends to increase inthe size of a pixel thereof, as the liquid crystal display unit iscorrespondingly increased in configuration. For instance, the size ofpixel of the conventional liquid crystal display unit was 0.2×0.2 (mm²).However, the present inventors have developed a liquid crystal displaydevice having a pixel size of 0.32×0.32 (mm²).

In this type of liquid crystal display device, foreign substances suchas dust or the like are intermixed in the liquid crystal display devicein the manufacturing process, or the foreign substances are adhered to amask for use with photolithography. If the foreign substances arepresent or intermixed in between the source electrode (or transparentpixel electrode) and the drain electrode of the thin film transistor,short-circuiting takes place between these electrodes, resulting in aso-called point defect in which the short-circuited pixel isdeteriorated. If the foreign substances are likewise present orintermixed in between the source electrode (transparent pixel electrode)and the gate electrode of the thin film transistor, the same pointdefect is caused. From this phenomenon, the present inventors have foundout such a problem that the point defect (e.g., a loss of pixel),inherent in the above-described liquid crystal display device, becomesconspicuous, as each individual pixel increases in size.

Incidentally, the arrangement that a configuration of the gate electrodeis made larger than the semiconductor layer has already been known inJapanese Patent Laid-Open Publication No. 17962/1985. However, even whensimply increasing the size of the gate electrode, a parasiticcapacitance between the gate electrode and the source electrode alsoincreases, and a DC component applied to the liquid crystal due toscanning signals is increased. In all, the undesirable results become soprevalent that utilization is difficult.

An example of an active matrix liquid crystal display device isdescribed on, e.g., pp. 193 to 200 of NIKKEI ELECTRONICS issued on Dec.15, 1986, published by Nikkei McGraw-Hill Co., Ltd.

The following listings are exemplary of the pixel dividing technique inthe active matrix liquid crystal display device: Japanese PatentLaid-Open Publication Nos. 49994/1982, 78388/1984, 97322/1985 and77886/1986.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a liquidcrystal display device capable of reducing deterioration inOFF-characteristics of a TFT due to light incident on the TFT.

To this end, according to one aspect of the invention, there is provideda liquid crystal display device capable of improving theOFF-characteristics of the TFT and restraining a DC component applied tothe liquid crystal.

According to another aspect of the invention, there is provided, in theliquid crystal display device, a technique capable of diminishing apoint defect which causes deterioration of pixels on a liquid crystaldisplay unit.

According to still another aspect of the invention, there is provided,in the liquid crystal display device, a technique capable of making ithard to visually perceive the point defect which appears on the liquidcrystal display unit.

According to a further aspect of the invention, there is provided, inthe liquid crystal display device, a technique capable of decreasing thepoint defect which causes the deterioration of the pixels on the liquidcrystal display unit and also reducing black scattering appearing on theliquid crystal display unit thereof.

According to a still further aspect of the invention, there is provided,in the liquid crystal display device, a technique capable ofaccomplishing the above-described objects, decreasing a resistance valueof scanning signal lines and reducing the point defect attributed toshort-circuiting between a pixel electrode of the pixel and the scanningsignal lines.

Another object of the invention is to provide a technique capable ofreducing the black scattering and preventing disconnection of theelectrodes of a holding (or storage) capacitance element for diminishingthe black scattering.

Another object of the invention is to provide, in the liquid crystaldisplay device, a technique capable of reducing the black scatteringwith a simple constitution.

Another object of the invention is to provide, in the liquid crystaldisplay device, a technique capable of reducing the DC component appliedto the liquid crystal of the liquid crystal display unit and diminishingthe black scattering.

Another object of the invention is to provide, in the liquid crystaldisplay device including color filters, a technique capable of reducingthe point defect which appears on the liquid crystal display unit andensuring positioning allowance dimensions with respect to eachindividual pixel on the liquid crystal display unit and each individualcolor filter for every color.

Another object of the invention is to provide, in the liquid crystaldisplay device, a technique capable of diminishing the point defectappearing on the liquid crystal display unit and decreasing theprobability that the point defect or a linear defect occurs on theliquid crystal display unit.

Another object of the invention is to provide, in the liquid crystaldisplay device, a technique capable of diminishing the point defectappearing on the liquid crystal display unit and improving an area (anopening rate) of the pixel electrode of every pixel on the liquidcrystal display unit.

Another object of the invention is to provide, in the liquid crystaldisplay device, a technique capable of enhancing resolution of a colorpicture.

Another object of the invention is to provide a technique capable ofaccomplishing the above-described objects and reducing an area of wiringor eliminating a multilayered wiring structure.

Another object of the invention is to provide, in the liquid crystaldisplay device, a technique capable of reducing the deterioration ofconnection between the thin film transistor and the pixel electrode.

Another object of the invention is to provide a liquid crystal displaydevice capable of enhancing the contrast.

The principal features of the present invention are described asfollows:

(1) A liquid crystal layer is sealed between a top-surface-side glasssubstrate (SUB2) on which a common electrode (ITO2) is formed and anunderside glass substrate (SUB1) on which a pixel electrode (ITO1) and aTFT (TFT1) are formed (FIGS. 1 and 2). Viewed from the liquid crystallayer, a gate electrode (GT) of the TFT is disposed in close proximityto the underside substrate (SUB1), while a semiconductor layer (AS) isspaced away therefrom. The gate electrode (GT) has a large size which issufficient to completely cover (when viewed from below) thesemiconductor layer (AS).

According to this constitution, OFF-characteristics of the TFT can beimproved, because the incident backlight passing through the undersidesubstrate (SUB1) does not reach the semiconductor layer (AS) on accountof its being shielded by the gate electrode (GT).

(2) The pixels are disposed in intersection regions defined by twoscanning signal lines and two image signal lines. The thin filmtransistor of the pixel selected by one of two scanning signal lines issplit into a plurality of segments. The thus divided thin filmtransistor is connected to a plurality of segments into which the pixelelectrode is split. A holding capacitance element is constructed in sucha way that the divided pixel electrodes serve as one electrode thereof,and the other of two scanning signal lines serves as the other electrodethereof by using it as a capacitance electrode line.

In this arrangement, only part of the divided portions of the pixelbecomes the point defect. Otherwise, the point defect will spread overthe entire pixel. It is therefore possible to diminish the point defectof the pixel and at the same time to improve a holding characteristic ofa voltage applied to the liquid crystal due to the holding capacitanceelement, resulting in a drop in the amount of black scattering.Particularly, the divided pixels contribute to diminution in pointdefect derived from the short-circuit between the gate electrode and thesource electrode or the drain electrode of the thin film transistor.Besides, the point defect attributed to the short-circuit between thepixel electrode and the other electrode of the holding capacitanceelement can be reduced. Consequently, the point defect created in partof the split portions of the pixels is small as compared with the areaof the entire pixel, whereby it is hard to visually perceive the pointdefect.

Light shielding effects are enhanced by broadening the gate electrode.On the other hand, there arises a reverse effect in which the DCcomponent applied to the liquid crystal becomes a problem because of anincrease in overlapping parasitic capacitance between the sourceelectrode and the drain electrode. This reverse effect can, however, bereduced by virtue of the holding capacitor.

(3) The scanning signal lines are composed of composite films obtainedby superposing a plurality of conductive layers on each other. The gateelectrode and the capacitance electrode line are each composed of singlelayered films each consisting of one conductive layer among thecomposite films.

Based on this construction, in addition to the above-described effects,it is feasible to decrease a resistance value of the scanning signallines and reduce the point defect due to the short-circuit between thepixel electrode and the scanning signal lines.

(4) Formed between one electrode of the holding capacitance element anda dielectric film thereof is a base layer composed of a first conductivefilm and a second conductive film which is formed thereon and has asmaller specific resistance value and a smaller configuration than thoseof the first conductive film. The above-described one electrode isconnected to the first conductive film exposed from the secondconductive film of the base layer.

Owing to this arrangement, it is possible to minimize the disconnectionof one electrode of the holding capacitance element, because oneelectrode of the holding capacitance element can surely be bonded alonga stepped portion caused by the other electrode thereof.

(5) The capacitance electrode line of the first stage or the final stageis connected to the common pixel electrode of the pixel.

In this arrangement, the capacitance electrode line of the first stageor the final stage and part of the conductive layer of an outsideextension wire may be formed into one united body, and the common pixelelectrode is connected to the outside extension wire. The scanningsignal lines can therefore be connected to the common pixel electrodewith a simple constitution.

(6) The capacitance electrode line or the scanning signal line of thefirst stage is connected to the scanning signal line or the capacitanceelectrode line of the final stage.

Based on this arrangement, the scanning signal lines and the capacitanceelectrode lines are all connected to a vertical scanning circuit, andhence a DC offset system (a DC cancel system) may be adopted. As aresult, the DC component applied to the liquid crystal can be reduced,thereby increasing a life span of the liquid crystal.

(7) In accordance with an embodiment III of the present invention whichis illustrated in FIG. 15A, light shielding films 1 and 2 are providedto fill up gaps formed between the pixel electrodes ITO1 through ITO3.

In this embodiment III, the problem that the light such as backlightleaks out through the gaps between the pixel electrodes can be almostobviated.

(8) In accordance with an embodiment IV of the present invention, lightshielding films 3 and 4 are electrically connected to an adjacentscanning line GL.

In the embodiment IV, capacitors may equivalently be formed between thelight shielding films 1 and 2 (adjacent scanning line) and therespective divided pixel electrodes.

These and other objects, features and advantages of the invention willbecome more apparent on reading the following detailed description withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating the principal portion of one pixel ona liquid crystal display unit of an active matrix color liquid crystaldisplay device in which embodiment I of the present invention isincorporated;

FIG. 2 is a sectional view taken substantially along the line II—II ofFIG. 1, illustrating the portion cut by this cutting-plane line andperipheral portions of a sealing portion;

FIG. 3 is a plan view showing the principal portion of a liquid crystaldisplay unit on which a plurality of pixels are disposed depicted inFIG. 1;

FIGS. 4 to 6 are plan views each showing the principal portion in apredetermined process of manufacturing the pixels depicted in FIG. 1;

FIG. 7 is a plan view illustrating the principal portion in a statewhere color filters are superposed on the pixels depicted in FIG. 3;

FIG. 8A is a plan view illustrating the principal portion of one pixelon the liquid crystal display unit of the active matrix color liquidcrystal display device in which an embodiment II of the presentinvention is incorporated, and FIG. 8B is a partially enlarged viewthereof;

FIG. 9 is an equivalent circuit diagram showing the liquid crystaldisplay unit of the active matrix color liquid crystal display device inwhich the embodiments I and II of the present invention areincorporated;

FIG. 10 is a plan view illustrating the principal portion of one pixelin a layout somewhat modified from that of the pixel depicted in FIG. 8;

FIG. 11 is an equivalent circuit diagram of the pixel depicted in FIGS.8 and 10, respectively;

FIG. 12 is a time chart showing a driving voltage of a scanning signalline based on a DC offset system;

FIGS. 13 and 14 are equivalent circuit diagrams each illustrating theliquid crystal display unit of the active matrix color liquid crystaldisplay device in which the embodiment II of the present invention isincorporated;

FIG. 15A is a plan view illustrating the principal portion of one pixelon the liquid crystal display unit of the active matrix color liquidcrystal display device in which an embodiment III of the presentinvention is incorporated; and

FIG. 15B is a plan view illustrating the principal portion of one pixelon the liquid crystal display unit of the color liquid crystal displaydevice of the active matrix color liquid crystal device in which anembodiment IV of the present invention is incorporated.

The constitution of the present invention will hereinafter be describedin combination with one embodiment in which the present invention isapplied to the active matrix liquid crystal display device.

In all the accompanying drawings which illustrate the embodiments, thecomponents having the same functions are marked with the like symbols,and their repetitive descriptions are therefore omitted herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment I)

Turning attention now to FIG. 1 (a plan view of the principal portion),there is illustrated one pixel on a liquid crystal display unit of anactive matrix color liquid crystal display device in which an embodimentI of the present invention is actualized. FIG. 2 shows a section cut bythe cutting-plane line II—II of FIG. 1. FIG. 3 (a plan view of theprincipal portion) illustrates the principal portion of the liquidcrystal display unit on which a plurality of pixels are disposeddepicted in FIG. 1.

As illustrated in FIGS. 1 through 3, in the liquid crystal displaydevice, the pixel including a thin film transistor TFT and a transparentpixel electrode ITO is formed on an inside (on the side of a liquidcrystal) surface of a lower transparent glass substrate SUB1. The lowertransparent glass substrate SUB1 is shaped with a thickness of, e.g.,approximately 1.1 (mm).

Each individual pixel is disposed in an intersection region (a regionsurrounded by four signal lines) defined by two adjacent scanning signallines (gate signal lines or horizontal signal lines) GL and two adjacentimage signal lines (drain signal lines or vertical signal lines) DL. Asdepicted in FIGS. 1 to 3, the plurality of scanning signal lines GLextending in the row-direction are disposed (or arrayed) in thecolumn-direction, while the plurality of image signal lines DL extendingin the column-direction are disposed (or arrayed) in the row-direction.

The thin film transistor TFT of every pixel is split into three (pluralnumbers) segments within the pixel, viz., this transistor TFT iscomposed of thin film transistors (divided thin film transistors) TFT1,TFT2 and TFT3. All of the thin film transistors TFT1 to TFT3 are shapedto virtually have the same size (the width is equal to a channellength). Each of the divided thin film transistors TFT1 to TFT3 isformed mainly of a gate electrode GT, an insulating film GI, and ani-type (intrinsic conductive type in which deterministic impurities arenot doped) amorphous Si semiconductor layer AS, and a source electrodeSD1 and drain electrode SD2. Note that the source/drain is originallydetermined by a bias polarity therebetween, but the source/drain is, itshould be understood, interchangeable during the operation, because thepolarity is reversed during the operation in a circuit of the titleddevice of this specification. In the following description, however, oneis fixedly expressed as a source, and the other a drain for convenience.

The gate electrode GT is, as fully illustrated in FIG. 4 (a plan view ofthe principal portion in a predetermined manufacturing process), formedto assume a T-shape (it branches off in the T-like configuration) inwhich to protrusively extend from the scanning signal line GL in thecolumn-direction (e.g., a vertical direction in FIGS. 1 and 4). Namely,the gate electrode GT is arranged to extend virtually in parallel withthe image signal line DL. The gate electrodes GT are also arranged toprotrusively extend to regions in which the respective thin filmtransistors TFT1 to TFT3 are formed. The gate electrodes of the thinfilm transistors TFT1 to TFT3 are formed into one united body (as acommon gate electrode) in continuation from the same scanning signalline GL. The gate electrode GT consists of a first single layeredconductive film g1 so as to minimize the possibility of forming a largestepped portion (i.e., a step-like configured protrusion) in the formingregion of the thin film transistor TFT. The formation of the firstconductive film g1 involves the use of, for instance, a chromium (Cr)film having a thickness of approximately 1000 (Å) on the basis ofsputtering.

It can be observed from FIGS. 1, 2 and 5 that the gate electrode GT isshaped to be sufficiently larger than the semiconductor layer AS tocompletely cover this layer AS (when viewed from below). Where abacklight such as a fluorescent lamp or the like is provided underneaththe substrate SUB1, the non-transparent Cr gate electrode GT overshadowsthe semiconductor layer AS, with the result that no backlight strikesupon the layer AS. Hence, a conductive phenomenon caused by theirradiation of light, i.e., the deterioration of OFF-characteristics, isnot likely to occur. In connection with an original size of the gateelectrode GT, this electrode GT has a minimum width (including anallowance for positioning the gate electrode and the source/drainelectrodes) required to span the source/drain electrodes SD1 and SD2;and a length thereof which determines a channel width W depends upon aratio with respect to a distance L (a channel length) between the sourceelectrode and the drain electrode, i.e., such a length is conditional ona factor of W/L which determines a mutual conductance gm.

The configuration of the gate electrode employed in this embodiment is,as a matter of course, more than the original one.

If the gate electrode is put into consideration in terms of only a gatefunction and a light shielding function as well, the gate electrode andthe line GL cooperate with a single layer to form one united body. Inthis case, as a non-transparent (or opaque) conductive material, Alcontaining Si, pure Al or Al containing Pd may be selected.

The scanning signal line GL consists of a composite film of the firstconductive film g1 and the second conductive film g2 superposed thereon.The first conductive film g1 of the scanning signal line GL is formed inthe same manufacturing process as that of the first conductive film g1of the gate electrode GT, and is also arranged to be integral therewith.Based on the sputtering technique, the second conductive film g2 isformed of, e.g., an aluminum (Al) film which is 2000 to 4000 (Å) inthickness. The second conductive film g2 serves to decrease a resistancevalue of the scanning signal line GL and is capable of increasing avelocity (a writing characteristic of information on the pixels) atwhich a signal on a scanning signal line is transmitted.

The scanning signal line GL is arranged such that the second conductivefilm g2 has a width smaller than that of the first conductive film g1.That is, the scanning signal line GL is formed so as to level thesurface of an insulating film GI superposed thereon, because a steppedconfiguration of the side wall may be moderated.

The insulating film GI serves as a gate insulating film for each of thethin film transistors TFT1 to TFT3. The insulating films GI are disposedon the gate electrode GT and the scanning signal line GL. The formationof the insulating film GI involves the use of, e.g., a silicon nitridefilm having a thickness of 3000 (Å) or thereabouts on the basis ofplasma CVD. As described above, the surfaces of the insulating films GIare levelled in the forming regions of the thin film transistors TFT1through TFT3 and of the scanning signal lines GL.

The i-type semiconductor layer AS is, as fully depicted in FIG. 5 (aplan view of the principal portion in a predetermined manufacturingprocess), employed as a channel forming region of each of the pluralityof divided thin film transistors TFT1 to TFT3. The i-type semiconductorlayers AS of the plurality of divided thin film transistors TFT1 to TFT3are formed into one united body within the pixel. Namely, each of theplurality of divided thin film transistors TFT1 to TFT3 of the pixel isformed in an insular region of one (common) i-type semiconductor layerAS. The i-type semiconductor layer AS is formed of an amorphous siliconfilm or a polycrystalline silicon film, the thickness of which isapproximately 1800 (Å).

The i-type semiconductor layer AS is formed in continuation from theSi₃N₄ gate insulating film GI by the same plasma CVD device, changingsupply gas components in such a manner that this layer AS is not exposedto the outside from the plasma CVD device. Similarly, d0 (FIG. 2) and anN⁺ layer in which p for ohmic contact is doped are successively formedwith a thickness of about 400 (Å). Subsequent to this step, thesubstrate SUB1 is taken out of the CVD device, the N⁺-layer d0 and thei-layer AS are subjected to patterning to form independent insularportions shown in FIGS. 1, 2 and 5 by employing a photo-processingtechnique.

Thus, by virtue of the arrangement that the i-type semiconductor layersAS of the plurality of divided thin film transistors TFT1 to TFT3 of thepixel are formed into one united body, the drain electrode SD2 common tothe thin film transistors TFT1 to TFT3 passes over the i-typesemiconductor layer AS once (in fact, a step equivalent to the filmthickness obtained by totaling the thicknesses of the first conductivelayer g1, the N⁺-type semiconductor layer d0 and the i-typesemiconductor layer AS) from the drain electrode SD2 to the i-typesemiconductor layer AS. This contributes to a drop in the probabilitythat the drain electrode SD2 is disconnected, which further leads to adecrease in the possibility of causing the point defect. In theembodiment I, the point defect created in the pixel when the drainelectrode SD2 goes over the step of i-type semiconductor layer AS can bereduced by a factor of 3.

Though different from the layout of the embodiment I, where the portionof the image signal line DL which has gone directly over the i-typesemiconductor layer AS is formed as the drain electrode 2, it ispossible to diminish the probability that a linear defect is caused dueto the disconnection when the image signal line DL (the drain electrodeSD2) goes over the i-type semiconductor layer AS. In other words, thei-type semiconductor layers AS of the plurality of divided thin filmtransistors TFT1 to TFT3 of the pixel are formed into one united body,whereby the image signal line DL (the drain electrode SD2) goes over thei-type semiconductor layer AS only once (in fact, however, twice—thebeginning and the end of such an action).

The i-type semiconductor layer AS, as depicted in detail in FIGS. 1 and5, extends to the intersection (a crossover) between the scanning signalline GL and the image signal line DL. The thus extended i-typesemiconductor layer AS is intended to diminish the degree ofshort-circuiting between the scanning signal line GL and the imagesignal line DL at the intersection.

The source electrode SD1 and the drain electrode SD2 of each of theplurality of divided thin film transistors TFT1 through TFT3 of thepixel are, as fully illustrated in FIGS. 1, 2 and 6 (a plan view of theprincipal portion in the predetermined manufacturing process), sodisposed on the i-type semiconductor layer AS as to be spaced away fromeach other. The source electrode SD1 and the drain electrode SD2 may beinterchangeable in terms of operation when the bias polarity of thecircuit varies. Namely, as in the case of an FET, the thin filmtransistor TFT is of a bidirectional type. Each of the source electrodeSD1 and the drain electrode SD2 is so arranged that a first conductivefilm d1, a second conductive film d2 and a third conductive film d3 aresequentially superposed from the lower layer contiguous to the N⁺-typesemiconductor layer do. The first, second and third conductive films d1,d2 and d3 of the source electrode SD1 are formed in the same process asthat of the drain electrode SD2.

The first conductive film d1 is composed of a chromium film shaped bysputtering, the thickness of which is 500 to 1000 (Å) (approximately 600(Å) in this embodiment). The chromium film has such properties that thestress increases, if the film thickness becomes large. Therefore, thethickness must fall within a range of about 2000 (Å). The chromium filmhas a favorable contact condition with respect to the N⁺-typesemiconductor layer d0. The chromium film also performs a function toprevent aluminum contained in the second conductive film d2 fromdiffusing into the N⁺-type semiconductor layer d0 by forming a so-calledbarrier layer. In addition to the chromium film, the formation of thefirst conductive film d1 may involve the use of a high melting pointmetal (Mo, Ti, Ta and W) film or a high melting point metal silicide(MoSi_(z), TiSi_(z), TaSi_(z) and WSi_(z)) film.

After the patterning has been effected on the first conductive film d1by the photo-processing, the N⁺-layer d0 is removed by the samephoto-processing mask or with the first conductive film d1 serving as amask. More specifically, the N⁺-layer do left on the i-layer AS exceptfor the first conductive film dl is removed by self-alignment. At thistime, the N⁺-layer d0 is etched so that the portion equivalent to itsthickness is all removed, and hence the surface of the i-layer AS isalso etched to some extent. The extent to which the surface is etchedmay be controlled according to the etching time.

Subsequently, the second conductive film d2 is formed of aluminum with athickness of 3000 to 4000 (Å) (approximately 3000 (Å) in thisembodiment) by sputtering. The aluminum film is smaller in stress thanthe chromium film and can be formed thick. The aluminum film behaves toreduce resistance values of the drain electrode SD2 and the image signalline DL. The second conductive film d2 is arranged to increase thevelocities at which the thin film transistor TFT functions and at whichthe signal of the image signal line DL is transmitted. Namely, thesecond conductive film d2 is capable of improving the writingcharacteristic of the pixel. Excepting the aluminum film, the secondconductive film d2 may be formed of an aluminum film containing silicon(Si) and copper (Cu) as additives.

After the second conductive film d2 has undergone patterning based onthe photo-processing technique, the third conductive film d3 is composedof a transparent conductive film (ITO: a nesa film) having 1000 to 2000(Å) (approximately 1200 (Å) in this embodiment) in thickness, whichrequires the sputtering process. The third conductive film d3constitutes not only the source electrode SD1, the drain electrode SD2and the image signal line DL but also the transparent pixel electrodeITO.

The first conductive films dl of the source electrode SD1 and of thedrain electrode SD2 are each shaped larger on the side of channelforming region than the upper second conductive film d2 and the thirdconductive film d3 as well. To be more specific, if there is createdsome deviation in mask alignment in the manufacturing process betweenthe first, second and third conductive films d1, d2 and d3, the firstconductive film d1 is arranged to become larger than the second andthird conductive films d2 and d3 (the channel forming regions of thefirst, second and third conductive films d1, d2 and d3 may be on theline). The first conductive films d1 of the source electrode SD1 and thedrain electrode SD2 are each so formed as to prescribe the gate length Lof the thin film transistor TFT.

In the plurality of divided thin film transistors TFT1 to TFT3 of thepixel, each of the first conductive films d1 of the source electrode SD1and the drain electrode SD2 is shaped larger on the side of channelforming region than the second conductive film d2 and the thirdconductive film d3 as well. This arrangement permits the gate length Lof the thin film transistor TFT to be defined by a dimension between thefirst conductive films d1 of the source electrode SD1 and of the drainelectrode SD2. The spacing (the gate length L) between the firstconductive films d1 can be prescribed by processing accuracy (patterningaccuracy), so that it is feasible to make uniform the gate length L ofeach of the thin film transistors TFT1 to TFT3.

The source electrode SD1 is, as explained earlier, connected to thetransparent pixel electrode ITO. The source electrode SD1 is formedalong the stepped portion (the step equivalent to the thickness obtainedby totaling the thicknesses of the first conductive film g1, theN⁺-layer do and the i-type semiconductor layer AS) of the i-typesemiconductor layer AS. More specifically, the source electrode SDlconsists of: the first conductive film d1 formed along the steppedportion of the i-type semiconductor layer AS; the second conductive filmd2 so formed thereon as to be smaller on the connecting-side to thetransparent pixel electrode ITO than the first conductive electrode d1;and the third conductive film d3 which is exposed from the secondconductive film d2 and is connected to the first conductive electroded1. The first conductive electrode d1 of the source electrode SD1 has agood bonding property with respect to the N⁺-type semiconductor layer doand is formed chiefly as a barrier layer against diffused matters fromthe second conductive film d2. The second conductive film d2 of thesource electrode SD1 is formed sufficiently dimensioned to extend overthe i-type semiconductor layer AS, because the chromium film of thefirst conductive film d1 cannot be formed too thick due to an increasein stress and is incapable of surmounting the stepped portion of thei-type semiconductor layer AS. That is, the second conductive film d2 isformed thick, thereby improving its step coverage. The second conductivefilm d2 which can be formed thick contributes greatly to a reduction inresistance value of the source electrode SD1 (this is the same with thedrain electrode SD2 as well as with the image signal line DL). The thirdconductive film d3 is incapable of surmounting the stepped portionassociated with the i-type semiconductor layer AS of the secondconductive film d2, and it follows that the third conductive film d3 isarranged to make a connection to the exposed first conductive film d1 byreducing the size of the second conductive film d2. The first and thirdconductive films d1 and d3 each have a favorable bonding property, andthe connecting portion therebetween is small. Hence, these twoconductive films can be securely connected to each other.

As discussed above, the source electrode SD1 of the thin film transistorTFT is composed of at least the first conductive film dl serving as thebarrier layer formed along the i-type semiconductor layer AS and thesecond conductive film d2 which is formed on the upper portion of thefirst conductive film d1 and has a smaller size and a smaller specificresistance value than those of the first conductive film d1. The firstconductive film d1 exposed from the second conductive film d2 isconnected to the third conductive film d3 defined as the transparentpixel electrode ITO, whereby the thin film transistor TFT can besecurely connected to the transparent pixel electrode ITO. It istherefore possible to reduce the point defect due to the disconnection.Besides, the source electrode SD1 may involve the use of the secondconductive film d2 (an aluminum film) having a small resistance value byvirtue of the barrier effects produced by the first conductive film d1,and this is conducive to a drop in resistance value.

The drain electrode SD2 is so formed as to be integral with the imagesignal line DL in the same manufacturing process. The drain electrodeSD2 assumes an L-like configuration wherein this electrode SD2 protrudesin such a row-direction as to intersect the image signal line DL. Thedrain electrode SD2 of each of the plurality of divided thin filmtransistors TFT1 to TFT3 of the pixel is connected to the same imagesignal line DL.

The transparent pixel electrode ITO is provided in every pixel andconstitutes one of the pixel electrodes of the liquid crystal displayunit. The transparent pixel electrode ITO is split into threetransparent pixel electrodes (divided transparent pixel electrodes)ITO1, ITO2 and ITO3 corresponding to the plurality of divided thin filmtransistors TFT1 to TFT3, respectively. The transparent pixel electrodeITO1 is connected to the source electrode SD1 of the thin filmtransistor TFT1. The transparent pixel electrode IT02 is connected tothe source electrode SD1 of the thin film transistor TFT2. Thetransparent pixel electrode ITO3 is connected to the source electrodeSD1 of the thin film transistor TFT3.

The transparent pixel electrodes ITO1 through ITO3 are, as in the caseof the thin film transistors TFT1 through TFT3, virtually of the samesize. Each of the transparent pixel electrodes ITO1 through IT03 is soformed as to be integral with the i-type semiconductor layer AS of eachof the thin film transistors TFT1 to TFT3 (the divided thin filmtransistors TFTs are concentrated on one portion), thus assuming theL-like configuration.

As is obvious from the description given above, the thin film transistorTFT of the pixel disposed in each of the intersection regions defined bythe two adjacent scanning signal lines GL and the two adjacent imagesignal lines DL is split into the plurality of thin film transistorsTFT1 to TFT3; and the thus divided thin film transistors TFT1 to TFT3are connected to the plurality of divided transparent pixel electrodesITO1 to ITO3. Owing to this arrangement, only part (for instance TFT1)of the divided portions of the pixel would be associated to contributingto the point defect, and hence there is no point defect in a largeproportion of the pixel (TFT2 and TFT3 are not associated with the pointdefect). Consequently, a magnitude of the point defect of the pixel canbe reduced on the whole.

The point defect created in part of the divided portions of the pixel issmall as compared with the entire area thereof (the point defect isone-third the area of the pixel in this embodiment), whereby it isdifficult to visually perceive the point defect.

Each of the divided transparent pixel electrodes ITO1 to ITO3 of thepixel is formed virtually of the same size. A uniform area of the pointdefect in the pixel can be obtained because of this arrangement.

Because each of the divided transparent pixel electrodes is formedvirtually of the same size, it is feasible to make uniform both a liquidcrystal capacitor (Cpix) provided by a combination of each of thetransparent pixel electrodes ITO1 to ITO3 and the common transparentpixel electrode ITO, and a superposition capacitor (Cgs) given bysuperposition of the transparent pixel electrodes ITO1 to ITO3 on thegate electrodes GT, this superposition capacitance being added to eachof the transparent pixel electrodes ITO1 to ITO3. Each of thetransparent pixel electrodes ITO1 to ITO3 can make uniform the liquidcrystal capacitance and the superposition capacitance, and it istherefore possible to make the DC component uniform which is applied toliquid crystal molecules of the liquid crystal LC due to thesuperposition capacitance. When adopting a way of offsetting the DCcomponent, scattering in the DC component applied to the liquid crystalof every pixel can be decreased.

Protection films PSV1 are provided on the thin film transistor TFT andthe transparent pixel electrode ITO. The protection film PSV1 is formedmainly for protecting the thin film transistor TFT from moisture or thelike. The protection film PSV1 should have high transparency and highmoisture-resistant properties. The protection film PSV1 is composed of,e.g., a silicon nitride film or a silicon oxide film formed by theplasma CVD, in which case the film thickness is approximately 8000 (Å).A light shielding film LS is disposed on the protection film PSV1 on thethin film transistor TFT, with the result that the light emerging fromthe outside does not strike upon the i-type semiconductor layer ASserving as a channel forming region. The light shielding film LS is, asdepicted in FIG. 1, disposed in the region surrounded by a dotted line.Based on sputtering, the light shielding film LS is formed of, e.g., analuminum film or a chromium film having high light shielding properties,the thickness of which is about 1000 (Å).

Therefore, it follows that the common semiconductor layer AS to the thinfilm transistors TFT1 through TFT3 is sandwiched in between therelatively large gate electrode GT and the light shielding films LSprovided up and down so as not to be irradiated with the outside naturallight or the beams of backlight. The light shielding film LS and thegate electrode GT are formed in a substantially similar configuration tothe semiconductor layer AS, but are larger than this semiconductor layerAS. The light shielding film LS and the gate electrode GT are almostequal in size (the gate electrode GT is depicted smaller than the lightshielding film LS to make the border line clear in the Figure).

Note that a backlight lamp may be installed on the side of the substrateSUB2, while the substrate SUB1 is provided as an observation side (anoutside exposing side). In this case, the light shielding film LSfunctions as a light shielding member against the backlight, while thegate electrode GT behaves as a light shielding member against thenatural light.

The thin film transistor TFT is arranged such that when applying apositive bias to the gate electrode GT, a channel resistance between thesource and the drain decreases, and if the bias becomes zero, thechannel resistance increases. The thin film transistor TFT serves tocontrol a voltage impressed on the transparent pixel electrode ITO.

The liquid crystal LC is sealed in an air space formed between the lowertransparent glass substrate SUB1 and the upper transparent glasssubstrate SUB2, the liquid crystal being prescribed by a lowerorientation film OR11 and an upper orientation film OR12 for orientingliquid crystal molecules.

The lower orientation film OR11 is formed on the upper portion of theprotection film PSV1 provided on the side of the lower transparent glasssubstrate SUB1.

Sequentially laminated on the inside (on the side of liquid crystal)surface of the upper transparent glass substrate SUB2 are a color filterFIL, the protection film PSV2, the common transparent pixel electrode(COM) ITO and the upper orientation film OR12.

The common transparent pixel electrode ITO stands vis-a-vis with thetransparent pixel electrode ITO provided in every pixel on the side ofthe lower transparent glass substrate SUB1, and cooperates with anotheradjacent common transparent pixel ITO to form one united body. Thiscommon transparent pixel electrode ITO is allowed to undergo impressionof a common voltage Vcom. The common voltage Vcom is defined as anintermediate electric potential between a low level driving voltageVdmin and a high level driving voltage Vdmax which are impressed on theimage signal line DL.

The color filter FIL is formed in such a manner that a dyeing basemember formed of resin, e.g., acrylic resin is stained with dyestuffs.For every pixel, the color filter FIL is disposed in a position standingvis-a-vis with the pixel. The color filters FIL are allocated accordingto the dyeing. Namely, as in the case of a pixel, each individual colorfilter FIL is disposed in the intersection region defined by the twoscanning signal lines GL and the two image signal lines DL. Each pixelis split into a plurality of segments in a filter of a predeterminedcolor of the color filter FIL.

The color filter FIL may be arranged in the following manner. Thearrangement begins with formation of the dyeing base member on thesurface of the upper transparent glass substrate SUB2. Excepting a redcolor filter forming region, the dyeing base member is then partlyremoved by the photolithography. Subsequent to this step, the dyeingbase member is stained with a red dyestuff and is subjected to a bondingprocess, thus forming a red filter R. Next, a green filter G and a bluefilter B are sequentially formed by performing the same processes.

The respective color filters of the color filter FIL are formed in theintersection regions so that these filters face the individual pixels.The scanning signal lines GL and the image signal lines DL each existbetween the respective color filters of the color filter FIL. Therefore,a space allowance, which corresponds to the presence of each signalline, for positioning can be ensured (a positioning margin can beenlarged). Moreover, when forming the individual color filters of thecolor filter FIL, a positioning space allowance between the differentcolor filters can also be ensured.

In accordance with this embodiment, the pixels are formed in theintersection regions defined by the two scanning signal lines GL and thetwo image signal lines DL. Each pixel is split into a plurality ofsegments, and the respective color filters of the color filter FIL areformed in such positions standing vis-a-vis with the thus dividedpixels. In this constitution, the above-described point defect can bediminished in magnitude, and at the same time it is feasible to ensurethe space allowance for positioning the respective pixels and the colorfilters.

The protection film PSV2 is designed for preventing the dyestuffs withwhich the color filter FIL is differently stained from permeating intothe liquid crystal LC. The protection film PSV2 is formed of, forexample, transparent resinous material such as acrylic resin, epoxyresin and so on.

The assembly of this liquid crystal display device involves the steps ofseparately forming layers on the side of lower transparent glasssubstrate SUB1 and the upper transparent glass substrate SUB2,superposing the lower and upper transparent glass substrates SUB1 andSUB2 on each other, and sealing the liquid crystal LC therebetween.

The plurality of pixels on the liquid crystal display unit are, asdepicted in FIG. 3, arranged in the same row-direction as the directionin which the scanning signal lines GL extend, thus constituting pixelrows X₁, X₂, X₃, X₄ . . . In each pixel of the pixel rows X₁, X₂, X₃, X₄. . . the positions in which the thin film transistors TFT1 to TFT3 andthe transparent pixel electrodes ITO1 to ITO3 are disposed are the same.To be more specific, in each pixel of the pixel rows X₁, X₃ . . . , thepositions in which the thin film transistors TFT1 through TFT3 aredisposed are set to the left, whereas the positions in which thetransparent pixel electrodes ITO1 through ITO3 are disposed are set tothe right. The individual pixels of the pixel rows X₂, X₄, . . . thatare positioned at the stage subsequent to the pixel rows X₁, X₃, . . .in the column-direction and the pixel of the pixel rows X₁, X₃, . . .each exhibit a linear symmetry with respect to image signal line DL. Ineach pixel of the pixel rows X₂, X₄, . . . , the thin film transistorsTFT1 to TFT3 are disposed on the right side, whereas the transparentpixel electrodes ITO1 to ITO3 are disposed on the left side. The pixelsof the picture element rows X₂, X₄, . . . are each placed to shift(deviate) a distance equivalent to half of a pixel in the row-directionwith respect to the pixels of the pixel rows X₁, X₃, . . . Supposingthat the intervals between the pixels of the pixel row X are all set to1.0 (1.0 pitch), the pixel interval is 1.0 in the next pixel row X, andhence the pixels deviate from those of the previous pixel row X with a0.5 pixel interval (0.5 pitch) in the row-direction. The image signallines DL disposed between the pixels and arrayed in the row-directionare such that each extends a distance equivalent to half of a pixel inthe row-direction between the pixel rows.

As discussed above, in the liquid crystal unit, the plurality of pixelsin which the thin film transistor TFT and the transparent pixelelectrode ITO are disposed respectively in the same positions arearranged in the row-direction, thus constituting the pixel row X. Thepixels of the next pixel row X and the pixels of the preceding pixel roware linearly symmetric with respect to the image signal line DL. Thepixels of the next pixel row are disposed to shift a distance in therow-direction equivalent to half of a pixel with respect to the pixelsof the previous pixel row. As illustrated in FIG. 7 (a plan view of theprincipal portion in a state where the pixels and the color filters aresuperposed on each other), it is therefore possible to provide a 1.5pixel interval (1.5 pitch) between each of the pixels of the previouspixel row X in which predetermined color filters are formed (forinstance, the pixels of the pixel row X₃ in which the red filters areformed) and each of the pixels of the next pixel row X in which the samecolor filters are formed (for example, the pixels of the pixel row X₄ inwhich the red filters are formed). The pixels of the pixel row X of theprevious pixel row X are disposed invariably at the 1.5 picture elementintervals from the pixels of the closest next pixel row in which thesame color filters are formed. The color filter FIL is allowed to take atriangular arrangement of RGB. This triangular arrangement of RGB of thecolor filter FIL is capable of enhancing conditions under which therespective colors are mixed. Hence, a resolution of color image can beimproved.

Between the pixel rows X, the image signal line DL extends a distancehalf of a pixel in the row-direction, whereby this image signal line DLdoes not intersect the adjacent image signal line DL. This eliminatesthe necessity of leading round the image signal line DL, resulting in adecrease in occupied area thereof. It is therefore feasible to eliminateboth a detour of the image signal line DL and the multilayered wiringstructure.

Directing attention to FIG. 9 (an equivalent circuit diagram of theliquid crystal display unit), there is illustrated a construction of acircuit of the liquid crystal display. In FIG. 9, the symbols YiG,Yi+1G, . . . indicate the image signal lines DL connected to the pixelsin which green filters G are formed. The symbols YiB, Yi+1B, . . .represent the image signal lines DL connected to the pixels in which theblue filters B are formed. The symbols Yi+1R, Yi+2R, . . . denote theimage signal lines DL connected to the pixels in which the red filters Rare formed. These image signal lines DL are selected by an image signaldriving circuit. The symbol Xi denotes the scanning signal line GL forselecting the pixel row X₁ depicted in FIGS. 3 and 7. Similarly, thesymbols Xi+1, Xi+2, . . . indicate the scanning signal lines GL forselecting the pixel rows X₂, X₃, . . . . These scanning signal lines GLare connected to a horizontal scanning circuit. Referring to FIG. 2, thecentral part thereof illustrates one pixel in section; the left partthereof illustrates a section, in which the outside extension wire isprovided, of the left fringes of the transparent glass substrates SUB1and SUB2; and the right part thereof illustrates a section, in which nooutside extension wire is provided, of the right fringes of thetransparent glass substrates SUB1 and SUB2.

Sealing materials SL shown on the right and left sides of FIG. 2 aredesigned for sealing the liquid crystal LC. The sealing materials SL areprovided along the entire fringes of the transparent glass substratesSUB1 and SUB2 except for a liquid crystal sealing port (notillustrated). The sealing material SL is formed of, e.g., epoxy resin.

The common transparent pixel electrode ITO on the side of the uppertransparent glass substrate SUB2 is connected leastwise at one portionto the outside extension wire formed of a silver paste material SIL onthe side of the lower transparent glass substrate SUB1. The outsideextension wire is formed in the same process as those of the gateelectrode GT, the source electrode SD1 and the drain electrode SD2.

Formed inside the sealing materials SL are layers of the orientationfilms OR11 and OR12, the transparent pixel electrode ITO, the commontransparent pixel electrode ITO, the protection films PSV1 and PSV2 andthe insulating film GI. Polarization plates POL are placed on the outersurfaces of the lower and upper transparent glass substrates SUB1 andSUB2.

(Embodiment II)

The embodiment II of the present invention is characterized by thefollowing points: an opening rate of each pixel on the liquid crystaldisplay unit of the liquid crystal display device is improved; and thepoint defect and the black scattering of the liquid crystal display unitare reduced by decreasing the DC component applied to the liquidcrystals.

FIG. 8A (a plan view of the principal portion) illustrates one pixel onthe liquid crystal display unit of the liquid crystal display device inwhich the embodiment II of the present invention is incorporated. FIG.8B is a view enlarged three times as large as the portion (TFT3 and itsperipheral portion), shown in FIG. 8A, surrounded by a bold solid frameline B on the lower left side in the Figure.

The liquid crystal display device of the embodiment II is arranged insuch a way that the i-type semiconductor layer As in each individualpixel on the liquid crystal display unit is, as illustrated in FIGS. 8Aand 8B, provided for each of the thin film transistors TFT1 throughTFT3. Namely, each of the plurality of divided thin film transistorsTFT1 through TFT3 is formed in an independent insular region of thei-type semiconductor layer AS.

In the thus constituted pixel, the thin film transistors TFT1 to TFT3can be equally allocated in the column-direction in which the imagesignal lines DL extend. Consequently, it is feasible to shape each ofthe transparent pixel electrodes ITO1 to ITO3 in a rectangularconfiguration and to connect them, respectively, to the thin filmtransistors TFT1 to TFT3. The transparent pixel electrodes ITO1 to ITO3(each assuming the rectangular configuration) serve to reduce an area ofspace (an area corresponding to the region indicated by the oblique lineshown in FIG. 8A is diminished) in the column-direction between thecontinuous transparent pixel electrode ITO within the pixel. As aresult, the improvement can be obtained in regard to the area (anopening rate).

As encircled by a dotted line marked with the symbol A, in FIG. 8A, avariation in configuration of each of the transparent pixel electrodesITO1 to ITO3 is made by using a line inclined at a certain angle to thescanning signal line GL or the image signal line DL (for example, a lineinclined at an angle of 45°). Each of the transparent pixel electrodesITO1 to ITO3 is capable of reducing the area of space between thetransparent pixel electrodes ITO as compared with a case where theconfiguration is varied by a line orthogonal to or parallel with thescanning signal line GL or the image signal line DL. Hence, the openingrate can be improved.

Each of the transparent pixel electrodes ITO1 to ITO3 is superposed onthe scanning signal line GL of the next stage in the column-directionboth on the side connected to the thin film transistor TFT and on theside opposite thereto. As in the case of the gate electrode GT of therespective thin film transistors TFT1 to TFT3, this superposition iseffected by causing the scanning signal line GL of the next stage tobranch off in a T-like shape, which is contiguous to the scanning signalline GL (the scanning signal line GL for selecting the pixel) forselecting its gate electrode GT. The thus diverged scanning signal lineGL is, as in the case of the gate electrode of the thin film transistorTFT, composed of a single layer of the first conductive film (chromiumfilm) g1. By virtue of the above-described superposition, there isconstituted a holding capacitance element (an electrostatic capacitanceelement) Cadd wherein each of the transparent pixel electrodes ITO1 toITO3 is employed as one electrode, and the portion diverged from thescanning signal line of the next stage which serves as a capacitorelectrode line is used as the other electrode. A dielectric film of theholding capacitance element Cadd is formed of the same layer as that ofthe insulating film used as a gate insulating film of the thin filmtransistor TFT.

As in the embodiment I, the gate electrode GT is formed larger than thesemiconductor layer AS. In this embodiment, however, the thin filmtransistors TFT1 to TFT3 are formed for every semiconductor layer AS,and hence a relatively large pattern is formed per thin film transistorTFT. Simultaneously, a connection to the diverged gate wire GL (g1) ismade.

FIG. 10 (a plan view illustrating the principal portion of one pixel inanother example) shows another layout of the holding capacitance elementCadd. Referring to FIG. 11 (an equivalent circuit diagram), there isdepicted an equivalent circuit of the pixel shown in FIGS. 8 and 10. Theholding capacitance element Cadd depicted in FIG. 10 exhibits anincrement in the holding capacitance by an enhancing of thesuperposition of each of the transparent pixel electrodes ITO1 to ITO3on the diverged portion (the other electrode of the holding capacitanceelement Cadd) of the capacitance electrode line. Fundamentally, theholding capacity element Cadd shown in FIG. 10 is identical with theholding capacitance element Cadd illustrated in FIG. 8. In FIG. 11, asin the previous case, the symbol Cgs represents the amount ofsuperposition associated with the source electrode SD1 and the gateelectrode GT of the thin film transistor TFT. The dielectric film of thesuperposition quantity Cgs is defined as the insulating film GI. Thesymbol Cpix designates a liquid crystal capacitor provided between thetransparent pixel electrode ITO (PIX) and the common transparent pixelelectrode ITO (COM). The dielectric film of the liquid crystal capacitorCpix includes the liquid crystal LC, the protection film PSV1 and theorientation films OR11 and OR12. The symbol V1c denotes a mid-pointpotential.

The holding capacitance element Cadd behaves to reduce the influence ofa gate potential variation ΔVg on the midpoint potential (a pixelelectrode potential) V1c. This will be expressed by the followingformula:

ΔV1c=(Cgs/(Cgs+Cadd+Cpix)×ΔVg

where ΔV1c is the amount of variation in the mid-point potential due toΔVg. This variation quantity ΔV1c is the cause of the DC componentapplied to the liquid crystal. A value of the variation quantity can bereduced as the holding capacitor Cadd is increased. The holdingcapacitance Cadd also has a function to increase the time of electricdischarge, whereby the image information after turning OFF the thin filmtransistor is unaltered. The reduction in the DC component applied tothe liquid crystal LC permits both improvement of life span of theliquid crystal LC and diminution in so-called seizing wherein thepreceding image still subsists when changing over the liquid crystaldisplay picture.

As discussed in the embodiment I, the gate electrode GT is large enoughto completely cover the semiconductor layer AS, and the area of overlapof the source electrode SD1 with the drain electrode SD2 increasescorrespondingly. Hence, a reverse effect is yielded in which theparasitic capacitor Cgs increases, and the mid-point potential V1c tendsto be adversely influenced by the gate (scanning) signal Vg. Thisnegative influence can, however, be obviated by providing the holdingcapacitor Cadd.

In the liquid crystal display device including the pixels disposed inthe intersection regions defined by the two scanning signal lines GL andby the two image signal lines DL, the thin film transistor TFT of thepixel selected by any one of the two scanning signal lines is split intoa plurality of segments. The thus divided thin film transistors TFT1through TFT3 are connected to the plurality of transparent pixelelectrodes (ITO1 through ITO3) in which the transparent pixel electrodeITO is split. Formed for each of the thus divided transparent pixelelectrodes ITO1 through ITO3 is the holding capacitance element Cadd inwhich the pixel electrode ITO serves as one electrode, and the otherscanning signal line GL of the two scanning signal lines, which isdefined as the capacitance electrode line, serves as the otherelectrode. In this arrangement, as explained earlier, only part of thedivided portions of the pixel becomes the point defect, and hence nopoint defect is caused in a large proportion of the pixel. It istherefore possible to reduce the magnitudes of the point defect and theDC component applied to the liquid crystal due to the holdingcapacitance element Cadd. This further leads to the improvement in thelife span of the liquid crystal LC. Especially, the division of pixelcontributes to a reduction in magnitude of the point defect caused froma short-circuit between the source electrode SD1 or the drain electrodeSD2 and the gate electrode GT of the thin film transistor TFT. Inaddition, it is feasible to diminish the point defect which would beattributed to a short-circuit between each of the transparent pixelelectrodes ITO1 to ITO3 and the other electrode (the capacitanceelectrode line) of the holding capacitance element Cadd. The latterpoint defect is decreased in magnitude by a factor of 3 in thisembodiment. As a result, the point defect produced in part of thedivided portions of the pixel is smaller than the entire area of thepixel, whereby the point defect is hard to be seen.

The holding capacity of the holding capacitance element Cadd is set to avalue which is 4 to 8 times the liquid crystal capacitor Cpix(4.Cpix<Cadd<8.Cpix) and 8 to 32 times the superposition capacitor Cgs(8.Cgs<Cadd<32.Cgs).

The scanning signal line GL is composed of the composite layer obtainedby superposing the second conductive film (aluminum film) g2 on thefirst conductive film (chromium film) g1. The other electrode of theholding capacitance element Cadd, viz., the diverged portion of thecapacitance electrode line, is formed of the single layer filmconsisting of a single layer of the first conductive film 9 of thecomposite film. Consequently, this arrangement is capable of decreasingthe resistance value of the scanning signal line GL and enhancing thewriting characteristic. Moreover, one electrode (transparent pixelelectrode ITO) of the holding capacitance element Cadd can securely bebonded to the upper portion of the insulating film GI along the steppedportion based on the other electrode of the holding capacitance element,thereby reducing the probability that one electrode of the holdingcapacitance element Cadd is to be disconnected.

The other electrode of the holding capacitance element Cadd isconstituted by a single layer of the first conductive film g1, but thesecond conductive film g2 defined as the aluminum film is not formed. Byvirtue of this arrangement, it is possible to prevent the short-circuit,which is due to the hillock of the aluminum film, between one electrodeand the other electrode of the holding capacitance element Cadd.

Formed between each of the transparent pixel electrodes ITO1 to ITO3which are superposed to constitute the holding capacitance element Caddand the diverged portion of the capacitance electrode line is an insularregion composed of the first conductive film d1 and the secondconductive film d2 as in the case of the source electrode SD1, with theresult that the transparent pixel electrode ITO is not disconnected whensurmounting the stepped portion of the diverged portion. This insularregion is shaped as small as possible so as not to diminish the area(opening rate) of the transparent pixel electrode ITO.

Disposed between one electrode of the holding capacitance element Caddand the insulating film GI employed as a dielectric film thereof is abase layer consisting of the first conductive film d1 and the secondconductive film d2 formed on this first conductive film d1, this secondconductive film d2 having a smaller size and a smaller specificresistance value than those of the first conductive film d1. Oneelectrode (a third conductive film d3) is connected to the firstconductive film d1 exposed from the second conductive film d2 of theabove-mentioned base layer, thereby making it possible to securely bondone electrode of the holding capacitance element Cadd along the steppedportion caused by the other electrode of the holding capacitance elementCadd. Therefore, the probability of an internal disconnection at thestepped portion of one electrode of the holding capacitance element Caddcan be reduced.

FIG. 13 (an equivalent circuit diagram showing the liquid crystaldisplay unit) illustrates a construction of the liquid crystal displayunit of the liquid crystal display device in which the transparent pixelelectrode ITO of the pixel is provided with the holding capacitanceelement. The construction of the liquid crystal display unit is based onrepetitions of a unit fundamental pattern including the pixel, thescanning signal line GL and the image signal line DL. The scanningsignal line GL of the final stage (or the scanning signal line of thefirst stage) used as a capacitance electrode line is, as depicted inFIG. 13, connected to the common transparent pixel electrode (Vcom) ITO.The common transparent pixel electrode ITO is, as illustrated in FIG. 2,connected to the outside extension wire through the silver pastematerial SIL on the fringe of the liquid crystal display device.Besides, some conductive layers (g1 and g2) of the outside extensionwire are formed in the same manufacturing process as that of thescanning signal line GL. As a result, this facilitates a connectionbetween the scanning signal line GL (capacitance electrode line) of thefinal stage and the common transparent pixel electrode ITO.

As explained earlier, since the capacitance electrode line of the finalstage is connected to the common transparent pixel electrode (Vcom) ITOof the pixel, the capacitance electrode line of the final stage can beso formed as to be integral with part of the conductive layers of theoutside extension wire. Furthermore, the common transparent pixelelectrode ITO is connected to the outside extension wire, and thecapacitance electrode line of the final stage is thereby connected tothe common transparent pixel electrode ITO with a simple arrangement.

Based on the DC offset system (DC cancel system) disclosed in JapanesePatent Application No. 62-95125 for which the present inventors appliedon Apr. 20, 1987, corresponding to U.S. Pat. No. 4,955,697, the liquidcrystal display device is capable of reducing the DC component appliedto the liquid crystal LC, as shown in FIG. 12 (a time chart), bycontrolling the driving voltage of the scanning signal line DL.Referring to FIG. 12, the symbol Vi represents a driving voltage of anarbitrary scanning signal line GL, and Vi+1 designates a driving voltageof the scanning signal line GL of the next stage. The symbol Veeindicates a driving voltage Vdmin which assumes a low level is impressedon the scanning signal line GL, and Vdd indicates a driving voltageVdmax which assumes a high level is impressed on the scanning signalline GL. Voltage variation quantities V₁ to V₄ of the mid-pointpotential (see FIG. 11) at the respective timings t=t₁ to t₄ are givensuch as:

t=t ₁ : ΔV1=−(Cgs/C)·V2

t=t ₂ : ΔV2=+(Cgs/C)·(V1+V2)−(Cadd/C)·V2

t=t ₃ : ΔV3=−(Cgs/C)·(V1+Cadd/C)·(V1+V2)

t=t ₄ : ΔV4=−(Cadd/C)·V1

However, a total pixel capacitance: C=Cgs+Cpix+Cadd.

If a sufficient driving voltage impressed on the scanning signal line GLis provided (see “Notes” given below), the DC voltage applied to theliquid crystal LC is expressed such as:

ΔV3+V4=(Cadd·V2−Cgs ·V1)/C,

hence,

Cadd·V2=Cgs·V=0

Then, the DC voltage applied to the liquid crystal LC comes to zero.

“Notes”: A variation quantity of a scanning line Vi exerts an influenceon the mid-point potential V1c at the timings t₁ and t₂. However, themid-point potential V1c becomes equal to the image signal potentialthrough a signal line Xi during a period of t₂ to t₃ (sufficient writingof the image signal). The potential applied to the liquid crystal issubstantially contingent upon a potential immediately after turning OFFthe thin film transistor TFT (a TFT OFF-period is sufficiently longerthan a TFT ON-period). Therefore, when calculating the DC componentapplied to the liquid crystal, a period of t₁ to t₃ may be almostignored, and what should be considered here is the potential just afterthe thin film transistor TFT has been turned OFF, i.e., the influenceproduced at the transition between the timings t₃ and t₄. It is to benoted that the polarity of the image signal Vi is inverted per frame orper line, and the DC component associated with the image signal itselfis zero.

Based on the DC offset system, an amount of decrease caused by thelead-in of the mid-point potential V1c due to the superpositioncapacitor Cgs is made to rise by the driving voltage impressed on thescanning signal line GL (capacitance electrode line) of the next stageas well as on the holding capacitance element Cadd, and the DC componentapplied to the liquid crystal LC can be minimized. This permits theliquid crystal display device to improve the life span of the liquidcrystal LC. As a matter of course, where the gate GT increases inconfiguration to enhance the light shielding effects, a value of theholding capacitor Cadd may be incremented correspondingly.

Adoption of this DC offset system may necessitate a step of, as shown inFIG. 14 (an equivalent circuit diagram illustrating the liquid crystalunit), connecting the scanning signal line GL (or the capacitanceelectrode line) of the first stage to the capacitance electrode line (orthe scanning signal line GL) of the final stage. In FIG. 14, only fourscanning signal lines are illustrated for convenience. In fact, however,several hundred pieces of scanning signal lines are disposed. Thescanning signal line of the first stage is connected to the capacitanceelectrode line of the final stage through an inside wire in the liquidcrystal display unit or the outside extension wire.

In the liquid crystal display device, as described above, the scanningsignal lines GL and the capacitance electrode lines are all connected toa horizontal scanning circuit by connecting the scanning signal lines ofthe first stage to the capacitance electrode lines of the final stage.Hence, the DC offset system (DC cancel system) is allowed to beutilized. As a result, the DC component applied to the liquid crystal LCcan be reduced, thereby improving the life span of the liquid crystal.

The present invention made by the present inventors has concretely beendescribed so far on the basis of the illustrative embodiments. Thepresent invention is not, however, limited to the above-describedprecise embodiments. As a matter of course, various changes ormodifications may be effected therein without departing from the spiritor the scope of the invention.

For example, in accordance with the present invention, each individualpixel on the liquid crystal display unit of the liquid crystal displaydevice can be split into two or four segments. If the number of dividedsegments of the pixel becomes too large, it follows that the openingrate goes down. As explained earlier, it is therefore adequate that thepixel be split into two or four segments. Even if the pixel is notdivided, however, the light shielding effects can be obtained. Theforegoing embodiment has presented a reverse stagger structure in whichthe formation is performed in the order of gate electrode→gateinsulating film→semiconductor layer→source and drain electrodes.However, another reverse stagger structure in which the up-and-downrelation or the sequence of formations are opposite to the former onesis also available in this invention.

(Embodiment III)

Referring to FIG. 15A, there is shown an improvement of the embodimentof FIG. 8A. The modified point is that light shielding films 1 and 2 areformed between the divided pixel electrodes. The light shielding films 1and 2 are formed of layers each assuming the same level as that of thefirst conductive film g1 employed for the scanning line GL and theelectrode of the capacitor Cadd and the gate electrode GT. However, thelight shielding films 1 and 2 are formed separately from the capacitorelectrode and the gate electrode, and are electrically arranged to be ina floating state. Provided that for instance, the photomask or theetching process is deteriorated due to undesirable conditions in themanufacturing process, no deterioration is created even when the lightshielding films are short-circuited to either the gate electrode GT orthe capacitor electrode owing to the foregoing floating state.

According to this embodiment, there are obtained the light shieldingeffects equal to the backlight shielding effects associated with thegate electrode. It is possible to considerably restrict an amount oflight leaking from gaps formed between the divided pixel electrodes ITO1to ITO3. In addition, the black display becomes more clear than everbefore, and this leads to enhancement of contrast.

(Embodiment IV)

The different point of an embodiment IV from the embodiment of FIG. 15Ais that light shielding films 3 and 4 are formed in continuation(electrically connected to) from the scanning signal line GL or theelectrode of the capacitor Cadd.

In this embodiment, the light shielding films 3 and 4 areshort-circuited to the gate electrode GT for the reason of theabove-described manufacturing process, in which case the two scanninglines are also short-circuited. The embodiment III is superior to thisembodiment in terms of eliminating such an undesirable condition.However, the following points are more advantageous than the embodimentIII.

(1) The scanning signal line GL is formed in continuation from itsdiverged line (capacitance electrode), which makes a spacingtherebetween unnecessary. Consequently, the amount of leaked light canbe further restricted.

(2) In combination with the light shielding effects, the capacitor Cadddescribed in the embodiment relative to the light can equivalently beformed between the pixel electrode and the adjacent scanning line; oralternatively a value of capacitance thereof can be increased.Therefore, if the value of auxiliary capacitor Cadd is kept constant,the opening rate becomes greater than in the embodiment III, whereby thedisplay becomes brighter.

Note that a total superposition area of the divided pixel electrodesITO1 to ITO3 and of the light shielding films 3 and 4 is made invariablein order to substantially equalize the values of respective auxiliarycapacitors Cadd. The superposition area of the two light shielding films3 and 4 and of the middle pixel electrode ITO2 overlapped with theselight shielding films 3 and 4 is almost half that of the pixelelectrodes ITO1 and ITO3 provided at both ends.

As discussed above, in the embodiments III and IV, the light leakingfrom the gaps between the divided pixels in the case of taking nomeasure is shielded by the light shielding films provided therebetween.Hence, there is yielded an effect of enhancing the contrast.

What is claimed is:
 1. A liquid crystal display device comprising apixel electrode and a thin film transistor, said thin film transistorincluding: a gate electrode, a semiconductor layer having a channelregion, and a source electrode and a drain electrode which areelectrically connected to said semiconductor layer, wherein saidsemiconductor layer has a step-like protruding configuration above aprincipal surface of a base layer, wherein said gate electrode is shapedto be larger than said semiconductor layer and is disposed over theplane surface of said base layer to completely cover said semiconductorlayer, wherein said source electrode is formed over said semiconductorlayer and has a step-like configuration substantially conforming withthe step-like protruding configuration of said semiconductor layer,wherein said source electrode includes a first conductive film and asecond conductive film superposed with said first conductive film atleast in a portion where said source electrode covers an edge portion ofsaid semiconductor layer created by the step-like protrudingconfiguration of said semiconductor layer, and wherein said pixelelectrode is connected to said source electrode in a portion where saidfirst conductive film is not superposed with said second conductivefilm.
 2. A liquid crystal display device according to claim 1, whereinthe first conductive film and the second conductive film are both formedover the step-like protruding configuration of the semiconductor layerand both include a step-like configuration substantially conforming withthe step-like protruding configuration of said semiconductor layer.
 3. Aliquid crystal display device according to claim 1, wherein said baselayer is a glass substrate.
 4. A liquid crystal display device accordingto claim 1, wherein said base layer is a glass substrate, and whereinsaid gate electrode is disposed on the principal surface of thesubstrate and said semiconductor layer is insulatedly disposed thereon.5. A liquid crystal display device according to claim 4, wherein saidpixel electrode is insulatedly disposed on the principal surface of thesubstrate.
 6. A liquid crystal display device comprising a pixelelectrode and a thin film transistor, said thin film transistorincluding: a gate electrode, a semiconductor layer having a channelregion, and a source electrode and a drain electrode which areelectrically connected to said semiconductor layer, wherein saidsemiconductor layer has a step-like protruding configuration above aprincipal surface of a base layer, wherein said gate electrode is shapedto be larger than said semiconductor layer and is disposed over theplane surface of said base layer to completely cover said semiconductorlayer, wherein said source electrode is formed over said semiconductorlayer and has a step-like configuration substantially conforming withthe step-like protruding configuration of said semiconductor layer,wherein said source electrode includes a first conductive film and asecond conductive film superposed with said first conductive film atleast in a portion where said source electrode covers an edge portion ofsaid semiconductor layer created by the step-like protrudingconfiguration of said semiconductor layer, and wherein the firstconductive film extends beyond an edge of the second conductive film andthe pixel electrode is connected to said source electrode at a portionwhere the first conductive film extends beyond the edge of said secondconductive film.
 7. A liquid crystal display device according to claim1, wherein the pixel electrode is also connected to the source electrodein a portion where the first conductive film is superposed with thesecond conductive film, such that the pixel electrode also includes astep-like configuration substantially conforming with the step-likeprotruding configuration of said semiconductor layer.
 8. A liquidcrystal display device according to claim 6, wherein the pixel electrodeis also connected to the source electrode in a portion where the firstconductive film is superposed with the second conductive film, such thatthe pixel electrode also includes a step-like protruding configurationof said semiconductor layer.
 9. A liquid crystal display deviceaccording to claim 6, wherein said base layer is a glass substrate. 10.A liquid crystal display device according to claim 6, wherein said baselayer is a glass substrate, and wherein said gate electrode is disposedon the principal surface of the substrate and said semiconductor layeris insulatedly disposed thereon.
 11. A liquid crystal display deviceaccording to claim 10, wherein said pixel electrode is insulatedlydisposed on the principal surface of the substrate.
 12. A liquid crystaldisplay device comprising a pixel electrode and a thin film transistor,said thin film transistor including: a gate electrode, a semiconductorlayer having a channel region, and a source electrode and a drainelectrode which are electrically connected to said semiconductor layer,wherein said semiconductor layer has a step-like protrudingconfiguration above a principal surface of a base layer, wherein saidgate electrode is shaped to be larger than said semiconductor layer andis disposed over the plane surface of said base layer to completelycover said semiconductor layer, wherein said source electrode is formedover said semiconductor layer and has a step-like configuration locatedabove the step-like protruding configuration of said semiconductorlayer, wherein said source electrode includes a first conductive filmand a second conductive film superposed with said first conductive filmat least in a portion where said source electrode covers an edge portionof said semiconductor layer created by the step-like protrudingconfiguration of said semiconductor layer, and wherein said pixelelectrode is connected to said source electrode in a portion where saidfirst conductive film is not superposed with said second conductivefilm.
 13. A liquid crystal display device according to claim 12, whereinthe first conductive film and the second conductive film are both formedover the step-like protruding configuration of the semiconductor layerand both include a stepped configuration located above the step-likeprotruding configuration of the semiconductor layer.
 14. A liquidcrystal display device according to claim 12, wherein said base layer isa glass substrate.
 15. A liquid crystal display device according toclaim 12, wherein said base layer is a glass substrate, and wherein saidgate electrode is disposed on the principal surface of the substrate andsaid semiconductor layer is insulatedly disposed thereon.
 16. A liquidcrystal display device according to claim 15, wherein said pixelelectrode is insulatedly disposed on the principal surface of thesubstrate.
 17. A liquid crystal display device comprising a pixelelectrode and a thin film transistor, said thin film transistorincluding: a gate electrode, a semiconductor layer having a channelregion, and a source electrode and a drain electrode which areelectrically connected to said semiconductor layer, wherein saidsemiconductor layer has a step-like protruding configuration above aprincipal surface of a base layer, wherein said gate electrode is shapedto be larger than said semiconductor layer and is disposed over theplane surface of said base layer to completely cover said semiconductorlayer, wherein said source electrode is formed over said semiconductorlayer and has a step-like configuration located above the step-likeprotruding configuration of said semiconductor layer, wherein saidsource electrode includes a first conductive film and a secondconductive film superposed with said first conductive film at least in aportion where said source electrode covers an edge portion of saidsemiconductor layer created by the step-like protruding configuration ofsaid semiconductor layer, wherein the first conductive film extendsbeyond an edge of the second conductive film and the pixel electrode isconnected to said source electrode at a portion where the firstconductive film extends beyond the edge of said second conductive film.18. A liquid crystal display device according to claim 12, wherein thepixel electrode is also connected to the source electrode in a portionwhere the first conductive film is superposed with the second conductivefilm, such that the pixel electrode also includes a step-likeconfiguration located above the step-like protruding configuration ofsaid semiconductor layer.
 19. A liquid crystal display device accordingto claim 17, wherein the pixel electrode is also connected to the sourceelectrode in a portion where the first conductive film is superposedwith the second conductive film, such that the pixel electrode alsoincludes a step-like configuration located above the step-likeprotruding configuration of said semiconductor layer.
 20. A liquidcrystal display device according to claim 17, wherein said base layer isa glass substrate.
 21. A liquid crystal display device according toclaim 17, wherein said base layer is a glass substrate, and wherein saidgate electrode is disposed on the principal surface of the substrate andsaid semiconductor layer is insulatedly disposed thereon.
 22. A liquidcrystal display device according to claim 21, wherein said pixelelectrode is insulatedly disposed on the principal surface of thesubstrate.